How to detect a black rectangle in specific area of image?

Hey everyone,

For my current project I want to track a moving black box in a video. But to start, I'd like to know how to do this in an image. I thought using HSV is the easiest, as in my case I'm trying to recognize pixels with [0 0 0] with a tolerance of [0.9 0.9 0.05]. However, I want to recognize this box inside of a specific frame within the image. After that, I want to know the coordinates of the center of gravity. I have tried using the colorDetectHSV script by Theodoros Giannakopoulos but I can't seem to get that one running either.

Does anyone know an easy way to do this?

Thanks in advance